368 research outputs found
Density Studies of MHD Interstellar Turbulence: Statistical Moments, Correlations and Bispectrum
We present a number of statistical tools for obtaining studying turbulence in
molecular clouds and diffuse interstellar medium. For our tests we used of
three-dimensional 512 cube compressible MHD isothermal simulations performed
for different sonic and Alfvenic Mach numbers. We introduce the bispectrum, a
new tool for statistical studies of the interstellar medium which, unlike an
ordinary power spectrum of turbulence, preserves the phase information of the
stochastic field. We show that the bispectra of the 3D stochastic density field
and of column densities, available from observations, are similar. We use the
bispectrum technique to define the role of non-linear wave-wave interactions in
the turbulent energy cascade. We also obtained the bispectrum function for
density and column densities with varying magnetic field strength. Larger
values of sonic Mach number result in increased correlations for modes with
different wavenumbers. This effect becomes more evident with increasing
magnetic field intensity. In addition to the bispectrum, we calculated the 3rd
and 4th statistical moments of density and column density, namely, skewness and
kurtosis, respectively. We found a strong dependence of skewness and kurtosis
with the sonic Mach number. In particular, as this number increases, so does
the asymmetry of the density distribution. We also studied the correlations of
2D column density with dispersion of velocities and magnetic field, as well as
the correlations of 3D density with magentic and kinetic energy and Alfven Mach
number for comparison. Our results show that column density is linearly
correlated with magnetic field for high sonic Mach number. This trend is
independent of the turbulent kinetic energy and can be used to characterize
inhomogeneities of physical properties in low density clumps in the ISM.Comment: apj emulated, 18 pages, 11 figures. Accepted to Astrophysical Journa
TurbuStat: Turbulence Statistics in Python
We present TurbuStat (v1.0): a Python package for computing turbulence
statistics in spectral-line data cubes. TurbuStat includes implementations of
fourteen methods for recovering turbulent properties from observational data.
Additional features of the software include: distance metrics for comparing two
data sets; a segmented linear model for fitting lines with a break-point; a
two-dimensional elliptical power-law model; multi-core fast-fourier-transform
support; a suite for producing simulated observations of fractional Brownian
Motion fields, including two-dimensional images and optically-thin HI data
cubes; and functions for creating realistic world coordinate system information
for synthetic observations. This paper summarizes the TurbuStat package and
provides representative examples using several different methods. TurbuStat is
an open-source package and we welcome community feedback and contributions.Comment: Accepted in AJ. 21 pages, 8 figure
Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients
The interstellar medium of the Milky Way is multi-phase, magnetized and
turbulent. Turbulence in the interstellar medium produces a global cascade of
random gas motions, spanning scales ranging from 100 parsecs to 1000
kilometres. Fundamental parameters of interstellar turbulence such as the sonic
Mach number (the speed of sound) have been difficult to determine because
observations have lacked the sensitivity and resolution to directly image the
small-scale structure associated with turbulent motion. Observations of linear
polarization and Faraday rotation in radio emission from the Milky Way have
identified unusual polarized structures that often have no counterparts in the
total radiation intensity or at other wavelengths, and whose physical
significance has been unclear. Here we report that the gradient of the Stokes
vector (Q,U), where Q and U are parameters describing the polarization state of
radiation, provides an image of magnetized turbulence in diffuse ionized gas,
manifested as a complex filamentary web of discontinuities in gas density and
magnetic field. Through comparison with simulations, we demonstrate that
turbulence in the warm ionized medium has a relatively low sonic Mach number,
M_s <~ 2. The development of statistical tools for the analysis of polarization
gradients will allow accurate determinations of the Mach number, Reynolds
number and magnetic field strength in interstellar turbulence over a wide range
of conditions.Comment: 5 pages, 3 figures, published in Nature on 13 Oct 201
Classification and identification of Pfiesteria and Pfiesteria-like species.
Dinoflagellates can be classified both botanically and zoologically; however, they are typically put in the botanical division Pyrrhophyta. As a group they appear most related to the protistan ciliates and apicomplexans at the ultrastructure level. Within the Pyrrhophyta are both unarmored and armored forms of the dominant, motile flagellated stage. Unarmored dinoflagellates do not have thecal or wall plates arranged in specific series, whereas armored species have plates that vary in thickness but are specific in number and arrangement. In armored dinoflagellates, the plate pattern and tabulation is a diagnostic character at the family, subfamily, and even genus levels. In most cases, the molecular characterization of dinoflagellates confirms the taxonomy on the basis of external morphology; this has been demonstrated for several groups. Together, both genetic and morphological criteria are becoming increasingly important for the characterization, separation, and identification of dinoflagellates species. Pfiesteria and Pfiesteria-like species are thinly armored forms with motile dinospore stages characterized by their distinct plate formulae. Pfiesteria piscicida is the best-known member of the genus; however, there is at least one other species. Other genetically and morphologically related genera, now grouped under the common names of "Lucy," "Shepherd's crook," and cryptoperidiniopsoid, are being studied and described in separate works. All these other heterotrophic dinoflagellate groups, many of which are thought to be benign, co-occur in estuarine waters where Pfiesteria has been found
JWST Constraints on the UV Luminosity Density at Cosmic Dawn: Implications for 21 cm Cosmology
An unprecedented array of new observational capabilities are starting to yield key constraints on models of the epoch of first light in the Universe. In this Letter we discuss the implications of the UV radiation background at cosmic dawn inferred by recent JWST observations for radio experiments aimed at detecting the redshifted 21 cm hyperfine transition of diffuse neutral hydrogen. Under the basic assumption that the 21 cm signal is activated by the Lyα photon field produced by metal-poor stellar systems, we show that a detection at the low frequencies of the EDGES and SARAS3 experiments may be expected from a simple extrapolation of the declining UV luminosity density inferred at z ≲ 14 from JWST early galaxy data. Accounting for an early radiation excess above the cosmic microwave background suggests a shallower or flat evolution to simultaneously reproduce low- and high-z current UV luminosity density constraints, which cannot be entirely ruled out, given the large uncertainties from cosmic variance and the faint-end slope of the galaxy luminosity function at cosmic dawn. Our findings raise the intriguing possibility that a high star formation efficiency at early times may trigger the onset of intense Lyα emission at redshift z ≲ 20 and produce a cosmic 21 cm absorption signal 200 Myr after the Big Bang
Effort required to finish shotgun-generated genome sequences differs significantly among vertebrates
<p>Abstract</p> <p>Background</p> <p>The approaches for shotgun-based sequencing of vertebrate genomes are now well-established, and have resulted in the generation of numerous draft whole-genome sequence assemblies. In contrast, the process of refining those assemblies to improve contiguity and increase accuracy (known as 'sequence finishing') remains tedious, labor-intensive, and expensive. As a result, the vast majority of vertebrate genome sequences generated to date remain at a draft stage.</p> <p>Results</p> <p>To date, our genome sequencing efforts have focused on comparative studies of targeted genomic regions, requiring sequence finishing of large blocks of orthologous sequence (average size 0.5-2 Mb) from various subsets of 75 vertebrates. This experience has provided a unique opportunity to compare the relative effort required to finish shotgun-generated genome sequence assemblies from different species, which we report here. Importantly, we found that the sequence assemblies generated for the same orthologous regions from various vertebrates show substantial variation with respect to misassemblies and, in particular, the frequency and characteristics of sequence gaps. As a consequence, the work required to finish different species' sequences varied greatly. Application of the same standardized methods for finishing provided a novel opportunity to "assay" characteristics of genome sequences among many vertebrate species. It is important to note that many of the problems we have encountered during sequence finishing reflect unique architectural features of a particular vertebrate's genome, which in some cases may have important functional and/or evolutionary implications. Finally, based on our analyses, we have been able to improve our procedures to overcome some of these problems and to increase the overall efficiency of the sequence-finishing process, although significant challenges still remain.</p> <p>Conclusion</p> <p>Our findings have important implications for the eventual finishing of the draft whole-genome sequences that have now been generated for a large number of vertebrates.</p
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